For catching fish which normally don't strike bait floating on the surface, or which usually remain well below the surface, fishermen use lures designed to dive beneath the surface. This category of fishing lure typically is buoyant and is designed to dive below the surface where the lure is pulled or trolled through the water. For example, a fisherman first casts out a diving lure, and then pulls in the lure by pulling up or "cranking" the tip of the fishing rod. The forward motion of the diving lure through the water generates a downward force on the lure, moving the lure down beneath the surface as the lure is pulled forwardly. The fisherman then lowers the tip of the rod and winds in the slack line, and then again cranks up the tip of the rod, further increasing the diving depth of the lure. Lures designed for fishing in this manner are called "crankbaits".
Crankbaits known in the prior art usually have a solid lure body supporting one or more fish hooks, with a diving plate extending forwardly from the front end of the lure body. These diving plates, sometimes called lips or bills, are angled downwardly relative to the lure body and water flowing over the diving plate exerts a downward diving force on the lure body as the lure is pulled or cranked forwardly through the water.
A crankbait, to be effective, should be capable of diving to desired fishing depths, preferably on the order of 20 feet or more. Moreover, the crankbait should be capable of diving to its greatest depth quickly, and without having to be fast-cranked through the water. Fast-cranking the lure brings the bait back to the angler far too quickly, and doesn't keep the lure in the effective fishing strike zone long enough to do much good. Prior art diving lures tried to accomplish these results with diving plates extending at a relatively large downward angle between the plate and the longitudinal axis of the lure body. The conventional wisdom was that the steeper the diving plate angled downwardly, the greater the diving depth resulting from each pull of the lure through the water. Other crankbait lure designs of the art include diving plates adjustably connected to the lure body, ostensibly to fine-tune the diving operation by modifying the overall effective length of the diving plate. One such lure is shown in U.S. Pat. No. 4,215,507. Other examples of prior-art diving lures are shown in U.S. Pat. Nos. 2,518,213; 2,542,447; 3,462,871; and 4,445,294.
Prior-art diving lures have a maximum diving depth bottoming out at about nine feet, despite extensive efforts to design lures exceeding that diving depth without fast-cranking the lure. This depth limitation of existing crankbaits greatly limits the effectiveness of the lure in a variety of fishing conditions. Where the water being fished is considerably deeper, e.g., 20-30 feet, than the maximum diving depth of existing crankbaits, many fish remain several feet below that maximum diving depth and those fish will not rise in the water to strike the lure. Moreover, modern injection-molded plastic crankbaits lack the natural counterweight effect of the metal-lipped crankbaits popular in the prior art. Crankbaits capable of diving beneath about nine feet were considered unusual, and the 20-foot diving depth was considered an absolute barrier for diving lures.